In the optical communications space, techniques used to detect data modulated onto an optical communications signal may be broadly grouped into two classes, namely “direct” detection and “coherent” detection. In “direct” detection techniques, the optical signal is made incident on a photodetector. The electrical current appearing at the photodetector output is proportional to the optical power which is the square of the optical Electric Field (E-Field) magnitude. Data modulated onto the optical signal power using an amplitude-modulation scheme, such as On-Off Keying (OOK) can thus be detected by analysis of the photodetector output current. Direct detection techniques have advantages in terms of low cost, and high reliability for On-Off Keying (OOK) based modulation schemes. As a result, the majority of optical receivers currently used in optical communications networks are based on direct detection.
In “coherent” detection techniques, the optical signal is mixed with a strong, narrow-line-width, local oscillator signal by an optical hybrid, and the combined signal made incident on one or more photodetectors. In some systems, the inbound optical signal is first split into orthogonal polarizations, and each polarization processed by a respective optical hybrid. In-phase and Quadrature components of each polarization can be detected using respective photodetectors positioned to receive corresponding signals output by the optical hybrid. The frequency spectrum of the electrical current appearing at the photodetector output(s) is substantially proportional to the convolution of the received optical signal and the local oscillator, and contains a signal component lying at an intermediate frequency that contains the data. Consequently, this “data component” can be isolated and detected by electronically filtering and processing the photodetector output current.
Coherent detection receivers offer numerous advantages over direct detection receivers, many of which follow from the fact that coherent detection techniques provide both phase and amplitude information of the optical signal. As such, more robust modulation schemes, such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and quadrature amplitude modulation (QAM) can be used.
However, receivers based on coherent detection techniques have suffered disadvantages that have, to date, prevented successful deployment in “real-world” installed communications networks. In particular, optical signals received through conventional optical links are distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Polarization effects of the fibre link tend to rotate the transmitted polarizations, so that, at the receiver, they will typically be neither orthogonal to each other nor aligned with the polarization beam splitter of the optical hybrid. As a result, each of the received polarizations (downstream of the polarization beam splitter) contain energy from both of the transmitted polarizations, as well as artefacts due to CD, PMD and PDL. These problems are compounded for polarization-division multiplexed signals, in which each transmitted polarization contains a respective different data signal. In such cases, each received polarization contains a mixture of both of the transmitted data signals, so that, in addition to compensating CD, PMD and PDL, it is also necessary to separate these data signals from one another.
Various methods have been proposed for addressing these problems. For example, a quadrature coherent receiver with electronic polarization compensation is described by R Noé, in: “Phase Noise-Tolerant Synchronous QPSK/BPSK Baseband-Type Intradyne Receiver Concept With Feedforward Carrier Recovery”, Journal of Lightwave Technology, Vol. 23, No. 2, February 2005, and “PLL-Free Synchronous QPSK Polarization Multiplex/Diversity Receiver Concept with Digital I&Q Baseband Processing”, IEEE Photonics Technology Letters, Vol. 17, No. 4, April 2005. In this respect, it will be noted that Noé also alludes (in the introduction) to the possibility of also compensating chromatic dispersion. However, Noé does not provide any teaching as to how this would be done. The applicability of RF channel estimation techniques to the detection of polarization-division multiplexed optical signals in a quadrature coherent receiver is described by Y. Han et al. in “Coherent optical Communication Using Polarization Multiple-Input-Multiple-Output”, OPTICS EXPRESS Vol. 13, No. 19, pp 7527-7534, 19 Sep. 2005.
FIG. 1 schematically illustrates the system of Noé (Supra, April 2005). As may be seen in FIG. 1, an optical signal received through an optical link 2 is divided by a polarization beam splitter 4 into orthogonal polarizations (nominally referred to as X and Y polarizations in FIG. 1), which are then mixed with a local oscillator (LO) 6 through a quadrature 90° optical hybrid 8. The composite optical signals appearing at the output of the optical hybrid are made incident on a set of photodetectors 10 to generate analog electrical signals Ix, Qx, Iy, Qy respectively corresponding to real (Re) and imaginary (Im) parts of each polarization. These analog signals are then supplied to a clock recovery circuit 12, before being sampled at the symbol rate by respective Analog-to-Digital (A/D) converters 14 to generate digital sample streams of each of the real (Re) and imaginary (Im) parts of each polarization. The digital samples are then supplied to a 1:M DEMUXer 16, which splits the data path into M parallel sample streams having a lower sample rate (by a factor of M), each of which is supplied to a respective processing module 18. Within each processing module 18, an inverse Jones matrix that models the polarization performance of the optical link is used to compensate polarization distortions. This function requires communications between each of the processing modules 18, as may be seen by the arrows in FIG. 1, so as to ensure continuity of compensation between each of the M sub-streams. The polarization compensated samples can then be decoded for data recovery.
In practical networks, the inbound optical signal can exhibit very high speed polarization transients. For example, polarization angle transients (rotations) at rates in excess of 2 KHz are common, and rotation rates in excess of 20 KHz have been observed by the inventors. Because of the high sensitivity of coherent detection systems to polarization angle, any receiver intended to be deployed in a real-world communications network, as opposed to a computer simulation or laboratory bench-top, must be able to track (that is, compensate) these transients.
A further limitation of coherent receivers is that a frequency mismatch between the received carrier and the local oscillator appears as a time varying phase error in detected symbols. When phase error reaches π/4 for QPSK or π/2 for BPSK, a “cycle-slip” can occur, in which symbols can be erroneously interpreted as lying in an adjacent quadrant. This can result in the erroneous interpretation of every symbol (and thus all data) following the cycle-slip. Typically, this problem is overcome by implementing a differential encoding/decoding scheme, in which each symbol is compared to its immediately preceding symbol, and the symbol value decided based on the difference. However, differential decoding has a disadvantage in that a symbol error results in two errored symbols; the symbol directly affected by the symbol error, and the symbol that immediately follows it. This doubles the raw bit error rate. It is also noted that due to the interaction of laser linewidth and noise, most cycle slip events occur in a duration of quite a few symbols (10 to 40 typically).
Prior art receiver systems do not offer a cost-effective means of addressing the above issues. For example, the system of Noé cannot track high speed transients of the type encountered in real-world communications networks. This is due, at least in part, to the slow speed (i.e. M/g symbol durations) at which the inverse Jones matrix coefficients can be updated. Thus, for example, Noé, claims that with a 10 GBaud signal (M=16 and g=10−4), the inverse Jones matrix coefficients can be updated with a period of 16 μs. This is far too slow to successfully track 20 KHz polarization rotations, which have a period of 50 μs. In addition, the system of Noé tends to fail in the presence of severe Chromatic Dispersion (CD), at least in part due to failure of the clock recovery circuit as inter-symbol interference (ISI) increases, and consequent uncertainty of the sample timing of the A/D converters. While it is mathematically possible to design a filter function that compensates both polarization and chromatic dispersion (as alluded to by Noé), the prior art does not offer any methods by which satisfactory compensation accuracy can be obtained with an adaptation speed high enough to track real-world polarization transients.
It is general practice for optical communications signals to include a periodic framing pattern, which normally defines a frame or packet comprising a payload for the transport of data, and an overhead containing addressing and timing information required to facilitate proper forwarding of the frame (or packet) as well as the insertion and extraction of payload data. The period of this frame pattern is chosen such as to not consume a significant fraction of the total symbols being transmitted. For example, the SONET OC-192 standard defines the A1 and A2 bytes that repeat at 8 KHz, which is a period of approximately 1,200,000 symbols. The G.709 OTN standard has framing periods of approximately 130,000 symbols. U.S. patent application Ser. No. 09/800,523, filed Mar. 8, 2001 describes a frame format that is tolerant to high error rates. The purpose of these frame patterns is to enable the detection of the location in time of the start of the frame so that the appropriate meanings can be assigned to the various byte locations within the frame, and then the appropriate parity and demultiplexing actions performed.
Packet protocols such as 10 G Ethernet define preambles in front of each packet to allow the identification of the start of the packet. These are identical for each packet.
All of the standard frame (packet) formats treat payload and overhead as a discrete unit. That is, data and overhead information is loaded into appropriate fields of the frame, and then transmitted as a unit to a receiver. Within the receiver, the received frame is buffered, and the content of each field read. None of the known framing techniques are directly applicable to physical (PHY) layer transport, for at least the reason that the meaning of each byte of the frame is determined by its location within the frame, and PHY-layer devices typically have no visibility of this information.
Accordingly, methods and techniques that enable efficient data transmission in an optical network, with cost-effective signal processing in a receiver unit remain highly desirable.